In the automobile industry, for example, vehicle bodies or parts thereof undergo treatment in which solvent can be released. The solvent can escape for example from surface coatings of the objects when these are dried in a drier. However, solvents are also already released when the objects are provided with paint or a different coating. Although, water-based coating materials have now become established, the coating materials used, and thereby particularly paints, frequently contain other solvents.
When paints are applied manually or automatically to objects, a substream of the paint, which generally contains both solid bodies and/or binding agents as well as solvent, is namely not applied to the object. This substream is known among experts as “overspray”. The overspray is taken up by the booth air which flows through the paint booth and supplied for separation as process air so that, if required, at least some of this process air can be conveyed back to the coating booth as circulating air after suitable conditioning.
Whilst the solid bodies and/or binding agent constituents or other components of the overspray are separated successfully by means of the separating systems which are known per se, be they wet separators or dry separators and, in particular, electrostatically operating separating systems here, solvent remains in the process air of the paint booth even after the separation process, which then arrives back in the paint booth with the circulating air.
Since the solvent concentration in the booth air would thereby continue to increase over time, the solvent concentration in the paint booth after a certain time would be greater than a maximum permissible upper threshold value which must not be exceeded, for example for explosion prevention reasons.
For this reason, only some of the solvent-containing process air is always supplied back to the paint booth as circulating air. The remaining fraction of booth air required for the air balance is provided by unladen fresh air.
However, the solvent concentration in the process air and therefore also in the circulating air conveyed back into the paint booth greatly depends, amongst other things, on the coating materials used, the application methods and the objects to be coated and can vary considerably.
In practice, the fresh air fraction is always selected to be great enough for the solvent concentration in the booth air to always remain safely below the permissible upper threshold value for the solvent concentration. In commercially known installations and processes, the fresh air fraction and the circulating air fraction of the booth air are always specified to be fixed and unchanging. The specified ratio of fresh air to circulating air here is set on the basis of empirical values and takes into account a safety margin so that the permissible upper threshold value for the solvent concentration in the booth air is never exceeded. However, it can arise here that the supplied fresh air fraction is much greater than would be necessary to observe the explosion prevention limits and the solvent concentration in the booth air therefore falls well below the permissible amount. In the most unfavourable case, all of the booth air is made up of fresh air for safety reasons and the solvent-containing process air is discharged completely as exhaust air and supplied for example to a thermal or regenerative post-combustion installation.
However, this is a waste of energy and resources since, on the one hand, too high a fraction of the process air is not reused and, on the other, the fraction of fresh air which is supplied is too high, and this likewise requires basic conditioning which uses energy.